We've just learned that the streetlight shining in your bedroom window every night is not only annoying; it may be jacking up your risk of breast cancer. Israeli researchers used satellite photos to gauge the level of nighttime light in 147 communities, then overlaid the photos with a map detailing the whereabouts of breast cancer cases. They discovered that women living in the brightest neighborhoods—with enough light to read a book outside at midnight—were 73 percent more likely to suffer from breast cancer than those residing in areas where most late-night light is celestial.

The brightest Israeli spots are comparable to any big American city, says Richard Stevens, PhD, one of the study's authors and a cancer epidemiologist at the University of Connecticut Health Center in Farmington, Connecticut. Stevens thinks electric light late at night—whether from street lamps or bathroom vanities—may factor into the threefold to fivefold difference in breast cancer rates between heavily industrialized and developing countries. "Something changes as societies industrialize," he says. "This might explain part of the increase."

The Israeli study isn't the first to link nocturnal light and breast cancer. Research has reliably demonstrated about a 50 percent rise in breast cancer among women who work the night shift compared with those who have day jobs. These findings fueled the World Health Organization's decision last winter to decree graveyard shift work a probable human carcinogen, putting it on the same footing as UV light and diesel exhaust.

Scientists aren't sure about the mechanism, but a likely culprit is the hormone melatonin. Potentially one of the body's most powerful cancer fighters, melatonin is made by the pineal gland and released in the middle of the night. If the gland doesn't detect sufficient darkness, it may not generate a healthy dose of the hormone. To ensure healthy melatonin levels, consider these measures:

Sleep in a dark bedroom. If you live in a city, invest in room-darkening shades. Even an eyeshade can help.

Use night-lights in the bathroom and hallways. Flicking on a lamp or overhead light can cause melatonin levels to drop immediately.

Try to go to bed at the same time every day, especially if you work the night shift. This will help keep your pineal gland on a healthy schedule.